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<h3 class="docSection1Title">2.4. Relationship of Standards and Implementations</h3>
<p class="docText">The standards that we've mentioned define a subset of any actual system. The focus of this book is on four real systems: FreeBSD 5.2.1, Linux 2.4.22, Mac OS X 10.3, and Solaris 9. Although only Solaris can call itself a UNIX system, all four provide a UNIX programming environment. Because all four are POSIX compliant to varying degrees, we will also concentrate on the features that are required by the POSIX.1 standard, noting any differences between POSIX and the actual implementations of these four systems. Those features and routines that are specific to only a particular implementation are clearly marked. As SUSv3 is a superset of POSIX.1, we'll also note any features that are part of SUSv3 but not part of POSIX.1.</P>
<p class="docText">Be aware that the implementations provide backward compatibility for features in earlier releases, such as SVR3.2 and 4.3BSD. For example, Solaris supports both the POSIX.1 specification for nonblocking I/O (<tt>O_NONBLOCK</tt>) and the traditional System V method (<tt>O_NDELAY</tt>). In this text, we'll use only the POSIX.1 feature, although we'll mention the nonstandard feature that it replaces. Similarly, both SVR3.2 and 4.3BSD provided reliable signals in a way that differs from the POSIX.1 standard. In <a class="docLink" href="ch10.html#ch10">Chapter 10</a> we describe only the POSIX.1 signal mechanism.</P>

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